The leading cause of lower back pain arises from rupture or degeneration of lumbar intervertebral discs. Pain in the lower extremities is caused by the compression of spinal nerve roots by a bulging disc, while lower back pain is caused by collapse of the disc and by the adverse effects of articulation weight through a damaged, unstable vertebral joint. One proposed method of managing these problems is to remove the problematic disc and replace it with a prosthetic disc that allows for the natural motion between the adjacent vertebrae (“a motion disc”).
U.S. Pat. No. 4,759,766 (“Buttner-Janz”) discloses one such motion device comprising three components: an inferior endplate, a superior endplate, and a core having two articulation interfaces. Both the inferior and superior endplates have raised bosses with concave spherical articulation surfaces in the center. The core has convex surfaces on both the top and bottom that are surrounded by raised rims. The articulation surfaces of the core are designed to articulate with the articulation surfaces of the endplates. A cross-section of this device is shown in FIG. 13.
Because articulating motion discs such as those described in Buttner-Janz seek to mimic the natural motion of the natural disc, it is desirable to place the disc at the precise location whereby the disc will have a center of rotation precisely equal to that of the natural disc. Accordingly, the device must be precisely placed at a predetermined spot during implantation in order mimic the natural center of rotation. However, it has been found that this is difficult to do in practice due to the limited visibility and space of the surgical field. For example, it has been reported that as much as 33% of the such discs are positioned at least 2 mm too anterior within the disc space. In addition, device movement may occur post operatively due to inadequate tissue adherence or trauma.
Although the surgeon can select a revision surgery to re-position the motion disc, such a surgery is costly and typically painful to the patient, and may include a risk of morbidity. Accordingly, it is an object of the present invention to allow post-implantation selective adjustment of the center of rotation of the articulating motion disc without requiring a revision surgery.